The performance of a ring laser gyroscope (RLG) is related to the lock-in band. This is a region of angular rate input around zero input rate in which the two oppositely directed traveling waves (ODTW) are synchronously locked due to mutual coupling therebetween. No conventional RLG output is obtained while the lock-in band and the scale becomes highly nonlinear as the lock-in band is approached. In one class of RLG a periodical varying bias (dither) is imposed on the RLG in order to minimize the effects of the lock-in region. Greater specifics are disclosed in U.S. Pat. No. 4,132,482, assigned to the present assignee. Although the patented approach operates generally satisfactorily, there are still nonlinearities in the scale factor and a random walk coefficient that are a function of the lock-in bandwidth.
In current RLG instrumentation a servo is employed to minimize a detectable signal known as the winking signal in order to reduce the lock-in band. However, it has been observed that in the case of some RLGs the lock-in is a minimum when the winking signal is a maximum. In others, the minimum lock-in bandwidth is found somewhere between minimum and maximum winking signals. The winking signals are ac modulation of the intensities of the laser beams that can be observed both within and outside of the lock-in bandwidth. Both the cw and ccw waves in the RLG exhibit the winking signals. This phenomenon arises from the backscatter that causes the coupling between two beams and results in the locking of two oscillators.
The relative phase between the winking signals can be readily observed in the laboratory by forming a Lissajou pattern with the two winking signals. This has been done and in general when the phase difference between the two winking signals is 180 degrees, the lock-in is minimized regardless of the winking signal amplitude. Investigation of the theory of RLGs tending to support this argument is presented hereinafter.
In most RLG instrumentation, the output is the phase difference between the optical oscillations of the two waves as observed in interference between the two waves. This is usually referred to as the "fringe pattern" and is to be retained. However, it is proposed that the device allowing the fringe pattern to be formed also provides for the observation of the individual intensities of the two beams.